Project Summary: We propose to develop fast spatial light modulators (SLM) to address the need for optical hardware compatible with the current fast genetically encoded sensors and actuators. SLMs are versatile optical components that enable beam steering and holographic projection of arbitrary patterns. They have recently been used for patterned optogenetic excitation of specific sets of neurons in the brain. However, current experiments are limited by the low speed of available spatial light modulators (~100 Hz). To match the needs of fast neuronal activation we propose to develop a technology for ultra-fast free-space SLMs with speeds exceeding 10 MHz operating at near infrared wavelengths (850nm-1000nm) and thus compatible with two-photon optogenetic excitation and two-photon microscopy. We already demonstrated proof of concept devices in silicon operating at telecom wavelength (~1550nm) modulated at slower speed via the thermo-optic effect. However, operation at telecom wavelengths is not suitable for most applications in neural stimulation and imaging, and Si can nor be used for wavelengths below ~1100nm because it is absorptive. We will use similar designs and techniques as in our current silicon photonics SLMs to develop devices in GaAs capable of fast optical beam steering in free space in the near infrared. The technology is based on GaAs nano-photonics that already enables on-chip photonic modulators operating at speeds exceeding several GHz. Aim 1: Develop proof of concept of a single SLM pixel and demonstrate operation at speeds faster than 10MHz in the near infrared. For this aim we will translate our already developed designs for Si SLMs to the GaAs material. We will show first modulation using the thermo-optic effect with speeds exceeding 1KHz, and then modulation using carrier injection/depletion at speeds exceeding 10MHz. The pixel size will be ~10mx10m Aim 2: Develop a prototype SLM with 8x8 pixels and demonstrate basic beam deflection functionality. This is a small version of the final device. It will allow us to troubleshoot any problems that may arise with the device, like cross-talk between pixels, heating, maximum optical power that can be handled. Aim 3: Develop a prototype SLM with >1000x1000 pixels and demonstrate fast optical beam steering. This is a SLM prototype that will have most of the functionality required to demonstrate proof of concept beam steering and pattern projection at speeds greater than 10MHz. We will demonstrate proof of concept two photon excitation microscopy.